I won't dispute with a medical man but from a veterinary angle we do recognise hypothyroidism and other metabolic diseases as predisposing to obesity.
However, it may be that Napoleon's calorie count crept up without him noticing - he may have thought he was only eating what he always ate but failed to notice that in times of prosperity he was eating it three times a day rather than once. A easy mistake to make and one which, combined with reduced physical activity, could easily have led to the visible weight increase.
I am surprised that it did not bother him, but perhaps he had that inbuilt belief that fatness indicated prosperity.

Susan